```

In [1]: import weakref                                                                                                    

In [2]: class A: 
   ...:     __slots__ = ()                                                                                                

In [3]: weakref.ref(A())                                                                                                  
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-60f73a442704> in <module>
----> 1 weakref.ref(A())

TypeError: cannot create weak reference to 'A' object

```